Cocaine use disorder remains a significant health problem in the United States, and effective and safe pharmacotherapeutic approaches are urgently needed to maximize treatment success and minimize lapses to drug use. The cycling course of cocaine use disorder is tied to a multitude of behavioral and cognitive processes with impulsivity (rapid unplanned reactions to stimuli without regard for the consequences) and cue reactivity (attentional bias toward cocaine-associated cues) cited as two key phenotypes that set up vulnerability to relapse even years into recovery. The serotonin (5-HT) system provides modulatory control over impulsivity and cue reactivity, particularly through the G protein-coupled 5-HT2C receptor (5-HT2CR). Data suggest that dampened 5- HT2CR signaling capacity may contribute to phenotypic vulnerability to relapse and that normalization of 5-HT2CR tone may be useful to suppress relapse promoted by impulsivity and cue reactivity. We hypothesize that a small molecule positive allosteric modulator (PAM) of the 5-HT2CR that augments the response to endogenous 5-HT and/or an exogenous 5-HT2CR orthosteric ligand is a novel strategy to restore 5-HT2CR function. The present grant is built upon our progress in the rational design, synthesis and pharmacological evaluation of new chemical entities based upon the only reported selective 5-HT2CR PAM PNU-69176E. We have synthesized new small molecules (e.g., CYD-1-79, CYD-3-30, CYD-6-16-2) which exhibit initial profiles as 5-HT2CR PAMs (functional signaling in live cells, radioligand binding assays) and reasonable oral and brain bioavailability. In vivo behavioral studies demonstrated that CYD-1-79, at doses that do not affect general motor activity, enhanced the effects of a selective 5-HT2CR agonist in drug discrimination analyses, and suppressed impulsivity and cue reactivity in rats, indicating efficacy in primary animal models pertinent to relapse in cocaine use disorder. Our objective is to optimize 5-HT2CR PAMs with a favorable drug metabolism and pharmacokinetics (DMPK) profile, and analyze select molecules in proof-of-concept behavioral models to support therapeutic potential for cocaine use disorder. To accomplish our objective, we will: (1) design, synthesize and optimize 5-HT2CR PAMs; (2) define selectivity and specificity and DMPK profiles of 5-HT2CR PAMs in vitro; and (3) determine DMPK in vivo and efficacy of optimized 5-HT2CR PAMs in rodent models of impulsivity and cue reactivity. This innovative, potentially high impact small molecule development project will elucidate important new information about the chemical neurobiology of 5-HT2CR allosteric modulation, and drive new concepts and directions in cocaine use disorder and anti-relapse medications.